1. Field of the Invention
The present invention relates to a substrate for information-recording media molded by injection molding, and a mold and a stamper for producing the same. In particular, the present invention relates to a plastic substrate preferably used for an information-recording disk capable of high density recording, a mold and a stamper to be used for injection molding for producing the same, a method for producing the stamper, and an information-recording disk such as a magnetic disk and an optical disk based on the use of the plastic substrate.
2. Description of Related Art
The age of multimedia has come. Accordingly, it becomes necessary, for information-recording devices used, for example, for general purpose computers, to deal with not only character information but also sound and image information. Therefore, it is required to further increase the storage capacity. In order to respond to this request, for example, it has been investigated to perform high density recording on the hard disk as an external recording device for the computer. A substrate made of an aluminum alloy such as Al-Mg has been hitherto used as a substrate for the hard disk. The substrate made of aluminum alloy is usually produced in accordance with the following process. At first, an aluminum plate, which is rolled to have a desired thickness, is subjected to press working to give a disk-shaped configuration. Distortion is removed therefrom by means of heat treatment, and then the inner and outer diameters are adjusted by means of cutting work. After that, the edge is subjected to chamfering work, and then the surface of the disk is subjected to electroless plating with Ni-P in a degree of 15 .mu.m to increase the surface hardness of the substrate. Finally, in order to obtain the smoothness and the flatness, the surface is subjected to lapping or polishing to finish it into a mirror-finished surface by using grinding abrasive grains such as alumina, silica, and diamond.
Recently, for example, in order to reduce the weight of the magnetic disk, improve the productivity, and decrease the production cost, it has been suggested that the substrate for the magnetic disk is produced in the same manner as the optical disk. For example, Japanese Patent Application Laid-Open No. 63-255816 discloses a method for producing a substrate made of resin for magnetic disks by injecting molten resin into a mold formed with ring-shaped minute recesses. The minute recesses are provided in order to reduce the torque which is applied upon starting of the rotation of the magnetic disk which adopts the contact start stop (CSS) system so that the magnetic disk is prevented from abrasion.
Conventionally, when the magnetic disk is subjected to physical format, it is necessary to record the format signal on the all area on the disk by using a magnetic head. Therefore, the conventional technique has required a lot of time. In order to deal with this problem, Japanese Patent Application Laid-Open No. 3-86912 discloses a magnetic disk as a new form of the magnetic disk, in which the preformat signal such as a servo signal is formed as a concave-convex structure on a resin substrate by means of injection molding, and a magnetic metal thin film is formed thereon. When the servo signal is read from the magnetic disk, the magnetic metal thin film is forcibly magnetized in one direction so that the leakage magnetic field, which is generated by the concave-convex structure of the magnetic metal thin film, is detected by using a magnetic head. The magnetic disk of this type can be produced by means of replication based on, for example, a method of injection molding by using a material of a master disk provided with accurately arranged tracking servo pits and guard band grooves for magnetically separating tracks from adjacent tracks. Therefore, a magnetic disk having a large capacity, on which physical format has been previously completed, can be provided cheaply in a large amount.
However, when an underlying layer or a magnetic layer is formed on the substrate by means of sputtering, a problem arises in that the substrate is deformed, and residual strain occurs, due to heating caused by the sputtering. In order to deal with this problem, for example, Japanese Patent Application Laid-Open No. 64-50236 discloses a method for producing a substrate for magnetic disks, in which injection molding is performed while setting the temperature of an injection molding mold to be at a temperature within a predetermined range lower than the glass-transition temperature of a resin used to produce the substrate, followed by annealing within a predetermined temperature range. Recently, a technique has been developed, which makes it possible to perform sputtering even at ordinary temperature. The problem concerning the deformation due to heating as described above is being gradually solved.
In order to increase the recording density of the magnetic disk, the near contact system is being developed, which makes it possible to decrease the floating amount of the recording magnetic head as small as possible. The near contact system allows the magnetic flux to be concentrated on a narrow range in the recording area. Therefore, it is possible to form minute magnetic marks at a high density. On the other hand, upon reproduction, the nearer the magnetic head approaches the magnetic disk surface, the larger the obtained reproduction output is. Therefore, it is desirable to realize the near contact system. For example, it has been known that when the floating height of the magnetic head from the magnetic disk surface is decreased from 40 nm to 20 nm, the reproduction output of the head is increased by about 20%.
In the case of the magnetic disk, in order to realize the near contact system, a floating head supported by a slider is adopted. The floating head of this type is operated as follows. That is, the air flow is thrust into a wedge-shaped gap formed between the slider and the magnetic disk in accordance with movement of the magnetic disk to generate a pressure thereby. The pressure allows the floating head to float over the disk rotating at a high speed, with an extremely narrow spacing distance intervening therebetween.
However, the substrate, which is molded by using the conventional injection molding mold, is insufficient in smoothness of the surface of the obtained substrate. For this reason, the floating amount of the floating head becomes high, making it difficult to realize the near contact system.
Recently, concerning the recording and reproduction apparatus for optical information-recording disks such as magneto-optical disks, the high speed access and the high speed recording have been investigated, which may be realized by embedding the optical head in the floating slider. For example, Japanese Patent Application Laid-Open No. 8-266369 discloses a magneto-optical disk recording and reproduction apparatus comprising a head provided with a solid immersion lens and a slider fixed at the tip of a sector-shaped swing arm. The recording and reproduction apparatus makes it possible to perform high speed access and high density recording, although it is extremely compact. The near contact system is also required for the optical disk such as the magneto-optical disk subjected to recording and reproduction by using such a floating type optical head, in the same manner as the magnetic disk.
Usually, the substrate for the optical disk such as the optical disk and the compact disk (CD) is produced by injection-molding a resin while installing a stamper formed with a concave-convex preformat pattern in a mold in an injection molding machine. However, when such a stamper is used, the stamper tends to be deformed during injection of the resin due to the heat of the molten resin and the pressure applied during the clamping process, as well as due to the difference in coefficient of thermal expansion between the stamper and the mold. The deformation of the stamper as described above brings about waviness and retardation of the substrate as a molded product.
In the case of the thin film type recording medium based on the use of the substrate made of resin described above, when the substrate is produced by injection molding by using the conventional stamper (almost all of available conventional stampers are made of pure Ni), the stamper made of Ni undergoes elastic deformation due to a large pressure (up to 500 kg/cm.sup.2) applied during the molding process, because the hardness of Ni is not more than 300 Hv as represented by Vickers hardness which is low, and the thickness is thin. The deformation (waviness) occurs as those having various cycles (wavelengths). The waviness having a cycle of waviness of not more than 10 mm has a height of waviness of not less than 50 nm. The elastic deformation of the Ni stamper is transferred to the substrate through the process of injection molding. The amount of deformation in such a degree has scarcely caused problem in the case of the conventional magneto-optical disk. However, in the case of the magnetic disk based on the use of the substrate and the magneto-optical recording medium of the new concept having the high density with the optical head embedded in the floating type slider to make high speed access, any head crash may occur during recording and reproduction, because the floating type slider has a small floating amount, and the floating type slider fails to follow minute waviness and irregularities of the substrate. Even when no crash occurs, problems arise in that the reproduction signal undergoes fluctuation or variation, and the tracking is not effected well, because the minute waviness exists on the substrate.
The new type magneto-optical recording system, in which information is recorded and reproduced by using the nearfield light as described above, involves the restriction that the propagation distance of the nearfield light is about 1/4 of the wavelength. For this reason, recording and reproduction should be performed while highly accurately positioning the floating type head with an extremely narrow spacing distance with respect to the magneto-optical recording medium. Therefore, the waviness of the substrate as described above is an extremely serious problem when it is intended to realize the recording and reproduction process by using the nearfield light.